1. Field of the Invention
This invention generally relates to methods and systems for detecting defects on a wafer by optical die to database inspection.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as ICs. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of semiconductor devices.
Many reticle inspection methods detect defects on reticles using die-to-database type comparisons. Such inspection typically involves acquiring a microscope image of a reticle. From a database that describes the intended pattern on the reticle, an image that the inspection microscope is expected to observe of that reticle may be calculated or simulated. The acquired optical image may then be compared to the calculated or simulated image to detect defects on the reticle. Such reticle inspection methods have proven useful for a number of uses. However, such reticle inspection methods are not capable of finding process-induced defects (i.e., defects that would be printed on a wafer due to the interaction between the reticle and the process of printing the reticle on the wafer).
Some reticle inspections are performed using wafers that have been printed with the reticles. In this manner, defects that are detected on the wafer can be used to determine if there are defects on the reticle that was used to print the wafer. Some such inspections are performed on optical platforms by comparing an inspected image frame to a reference frame, where the reference frame is a sample of the image generated from the wafer. Examples for reference image frames are: images from adjacent dies, images from a standard reference die on the same wafer or a different wafer, and images from the adjacent cells (in an array structure).
Currently, die to database inspection performed for wafers exists only on a scanning electron microscope (SEM) inspection platform. However, due to the throughput constraints (e.g., due to the physics of electron beam tools), only a substantially small number of locations (i.e., not the entire wafer and not entire dies on the wafer) can be checked. In addition, inspection performed by electron beam inspection of wafers is too slow to qualify every reticle for which qualification is needed. Furthermore, with the advent of multiple patterning step lithography process, and as a result needing multiple reticle qualifications for a single lithography process, the number of reticles for which qualification must be performed should grow substantially.
The currently available optical inspection methodologies that involve comparing wafer images to a reference wafer image to detect defects on a wafer cannot serve some of the use cases for which such inspection is performed. For example, such currently used optical inspections cannot detect repeater defects within dies printed with a single die reticle. One example of such a use case is for extreme ultraviolet (EUV) mask qualification. In particular, due to the lack of a pellicle, particles on the mask when printed on the wafer become repeater defects on the wafer. Therefore, such defects will cancel each other out in die-to-die comparisons and not be detected. In addition, such currently used optical inspections cannot be used for design intent checks. For example, a reference image generated from part of a wafer contains the process variation. Therefore, comparison of such a reference image with a different wafer image will cancel out the process variation in both images rendering the process variation undetectable. Furthermore, before the process becomes mature, it is difficult to find a “golden” reference die. For example, a user may have no idea which die or dies can be used as a “golden” reference die for comparison with other dies on a wafer.
Accordingly, it would be advantageous to develop systems and/or methods for detecting defects on a wafer that do not have one or more of the disadvantages described above.